Part 1: Python Basics with REPL

In the first part of this exercise, we will practice using several of the key Python concepts covered in the previous lesson. We will begin by launching REPL - the python3 interactive shell.

1. From the dock at the bottom of the screen, click the Terminal icon to open a Terminal window.
   
2. At the command prompt, type python3 and press Enter to open the python3 interactive shell (REPL).
   
   Take note of the Python version.
   
3. In REPL, type my_integer = 1 and press Enter to assign the number 1 to the my_integer variable, then type my_integer and press Enter to display the my_integer variable.
   
   You should see that REPL prints the current value of my_integer to the screen.
   
   Note: "variable=integer" and "variable = integer" are the same in Python. However, "variable = integer" is preferred and easier to read.
   
4. Execute the following commands:
   my_integer * 10
   my_integer + my_integer
   my_integer ** 3
   Note that the value of my_integer does not change - only the output does.
5. Execute my_integer = my_integer * 10 to reassign the value of my_integer from 1 to 10, then execute my_integer to confirm.
   
6. Execute type(my_integer) to use Python's type() function on my_integer. 
   
   The type() function reports what a variable is (e.g., integer or string). We see that my_integer is, in fact, "class int" - meaning integer.
   
7. Execute my_integer = 1.234 to update the value of my_integer, then execute my_integer to confirm.
   
8. Execute type(my_integer) to use Python's type() function on my_integer.
   
   Notice that the type has changed to float. You may recall that integers are whole numbers (no decimal values). A float is a floating point number - meaning it has decimals.
   
   The lesson here is that Python is dynamically typed. Variables are assigned their type at runtime. We do not need to assign variable types (a.k.a. cast variable types) in advance.
   
   Note: Naming a variable my_integer does not make it an integer.
   
9. Execute my_integer == 1 to compare my_integer to the number 1.
   
   Because 1 does not equal 1.234, Python reports back False. Try executing my_integer == 1.234 to see Python report back True.
   
   As discussed in the previous lesson, the symbols we use on variables ( +, ** , =,  ==,  etc. ) are called operators. Python operators are used for arithmetic, assignment, comparison, and membership.
   

   

   For a comprehensive overview of Python operators, see https://www.w3schools.com/python/python_operators.asp
   
10. Execute my_string = "Happy Puppy" to create a string, then execute my_string to confirm.
    
    In Python, a string is a character list. Each letter is placed into a special container called a list. We can access every element of a list by its index number:
    

    

    In my_string above, there are 11 elements (in this case, characters) starting at index 0.
    
11. Execute my_string[0] to display the first element in the list.
    
12. Execute the following commands:
    my_string[1]
    my_string[4]
    my_string[10]
    Take note of the output from my_string[10].
13. Execute the following command:
    my_string[11]
    Notice you get an error because there is no 11th element.
    
    When working with strings (and other lists), it's good to find the length of the string to avoid errors like the one above. Python provides the len() function just for that.
    
14. Execute len(my_string) to display the length of my_string.
    
    Remember, 11 means 0 to 10!
    
    Because strings are lists in Python, they can be chopped into their elements. Python provides a neat tool for chopping strings called slicing.
    
15. Execute my_string[0:5:1] to instruct Python to slice the string as follows:
    
    -Start at index 0.
    -Go to index 4 (slices are always index number -1).
    -Advance one element at a time (this is the default).
    
16. Execute the following commands:
    my_string[1:5:1]
    my_string[6:11:1]
    Did the second slice confuse you? There is no 11th element! Why did we not get an error? When slicing, we have to reference what we want less one. Thus, if we want to see the 10th element, we must ask for the 11th.
    
    There is likely a reasonable explanation for this silliness, but sometimes you just have to accept the rules, even arbitrary ones, to get a program to do what you want.
    
17. Press CTRL + L to clear the interactive terminal, then execute the following code:
    str1 = "the"
    str2 = "cow"
    str3 = "says"
    str4 = "moo"
    str5 = " "
    str6 = str1 + str5 + str2 + str5 + str3 + str5 + str4
    str6
    Adding strings like in the example above is called concatenation.
    
18. Press CTRL + L to clear the interactive terminal, then execute the following code:
    var1 = "1"
    var2 = "2"
    var3 = "3"
    var4 = var1 + var2 + var3
    var4
    Did you expect var4 to be 6? What type of variable is var1, var2, and var3? Are we dealing with addition (integers) or concatenation (strings)? Because the variables are characters (assigned as "1" rather than 1), Python will concatenate them, not add them.
    
19. Press CTRL + L to clear the interactive terminal, then execute the following code:
    str1 = "Pepper"
    str2 = "pepper"
    str1 == str2
    In Python, you can compare strings like you can numbers.
    
    Are the two strings above equal? When comparing strings, know that Python is case-sensitive.
    
20. Press CTRL + L to clear the interactive terminal, then execute the following code:
    str1.lower() == str2.lower()
    In the example above, we access the .lower() method of the string object to make str1 and str2 lowercase for the comparison operator.
    
    Note that the value of str1 does not change when we access a method. If we wanted to change str1, we would use:
    str1 = str1.lower()
    We have already explored lists when we discussed strings. Let's learn more about lists and explore the program logic known as a loop.
    
    Note:  Lists are known as arrays in other programming languages.
    
21. Press CTRL + L to clear the interactive terminal, then execute the following code:
    my_list = ["cat","dog","cow","pig","bunny","horse"]
    my_list
    my_list[0]
    my_list[1]
    len(my_list)
    How many elements are in my_list according to len()?
    
    Next, let's loop through my_list using a for loop.
    
    Note: Don't confuse a string (list of characters) with a list of strings. In a list of strings, each string is an element. In a string, each character is an element.
    
22. Type for animal in my_list: and press Enter.
    
    Notice the change in prompt. REPL is waiting for more instructions.
    
23. Indent in two spaces, then type print(animal) and press Enter twice.
    
    Pressing Enter at . . . tells REPL we are done typing commands and would like the interactive shell to run the program.
    

    

    In the example above, the for loop creates an animal variable, then steps through each element of the list and places that element into animal (one at a time). The loop then prints animal to the screen for each element.
    
24. Press CTRL + L to clear the interactive terminal, then execute the following code:
    car_makers=["Ford","Chevy","BMW","Volkswagen","KIA","Volvo"]
    for car_maker in car_makers:
    if car_maker == "BMW":
    print("Better make way!")
    else:
    print("Here comes a " + car_maker)
    Note there are two spaces before if and else and four spaces before the print statements. Proper indentation is critical in Python. Refer to the screenshot below to validate your input.
    

    

    The code above introduces a new logic element: the if / else condition. We test the current value of car_maker against the string "BMW".  IF there is a match, we print "Better make way!" ELSE, we concatenate "Here comes a " with the current string in the variable car_maker.
    
    Note: You will see this pattern repeatedly in programming: “for apple in apples”, “for cow in cows”, “for people in list_of_people”, etc. While “for x in y” will work just as well, it conveys no meaning to the reader. It’s best to use meaningful variables.
25. Execute exit() to leave the interactive Python terminal.

Part 2: Python Scripting with VS Code

While working with REPL interactively is great for testing syntax and quick prototyping, writing anything beyond a few lines becomes cumbersome and error-prone. You may have already discovered how easy it is to make a typo and have to start over. In the next part of this exercise, we will work in Microsoft's Visual Studio Code (VS Code) integrated development environment (IDE) to make our lives easier.

1. Execute code to launch VS Code.
   
   It will take 30-60 seconds for VS Code to launch.
   
2. From the VS Code menu bar, select File > Open Recent > ~/names.py to open the names.py script.
   

   

   In the names.py script, we see a list called names with names in it.
   
   We then see something new: the input() function. The input() function will prompt the user for input and then place that input into a variable.
   
   Finally, we see the now familiar for loop and if condition, used to test if name_requested matches any of the names in the list.
3. In the upper-right corner, click the Run icon (the triangle) to run the program.
   
   A new terminal window will open in the bottom half of VS Code.
   

   

4. At the "What name can I find for you:" prompt, type Karen and press Enter.
   
5. Click the Run icon to re-run the script. This time, type Zelda and press Enter.
   

   

   You should see that nothing happens. There's nothing in the script to handle a situation when a name is not found. Let's change that.
6. Alter the script by adding the following line of code to the bottom, then press CTRL + S to save.
   print("Sadly I did not find " + name_requested  + ".")

   

7. Click the Run icon to re-run the script. Once again, type Zelda and press Enter.
   

   

   That seems okay, but let's try the script with a name in the list (e.g., Mary).
   
8. Click the Run icon to re-run the script, then type Mary and press Enter.
   

   

   Our script is giving us conflicting messages. There are several possible solutions, but the simplest would be to end the script if a name is found. 
9. To implement this change, add exit() to the if condition, as shown below:
   

   

10. Press CTRL + S to save, then use the run icon to re-run the script. Try both Karen and Zelda.
    

    

    Great, but what if the user is lazy and does not want to capitalize the first letter in a name? 
    
11. Re-run the script and try karen.
    

    

    Recall that we can use the .lower() method on strings to make them all lowercase for testing.
12. Alter the script by adding the .lower() method to the name variables.
    

    

13. Press CTRL + S to save, then use the run icon to re-run the script. Try both karen and zelda.
    

    

    What if the program could add a name to the list of names? Happily, lists are mutable, meaning they can be altered.
14. Add the following code to the bottom of the name.py script:
    add_name = input("Would you like to add " + name_requested + " to the list of names? (Y/N)")
    
    if add_name.lower() == "y":
    names.append(name_requested)
    print(name_requested + " has been added to my list")
    else:
    print("Ok. I will not add " + name_requested + ".")
    exit()

    

15. Try running the script, searching for Zelda, then adding Zelda to the list. Then re-run the script and search for Zelda again.
    

    

    What's going on? Is our code even adding Zelda to the list? Let's confirm if Python is indeed appending Zelda to the names list.
    
16. Add the following code to the if condition, right above the else line.
    print(names)

    

17. Press CTRL + S to save, then re-run the script. Try adding Zelda to the names list.
    

    

    Zelda is being added to the names list, but if we re-run the script, we still find no Zelda.

    

    This is because the names list gets re-written every time the program runs. If we want our list to be stateful (to remember things), we must store the names in the list outside the script.
    
    One standard solution is to use a file. Fortunately, it is very easy to read and write files using Python. In the next steps, we will alter the script to read a text file with our names and then write any changes to a file before exiting.
    
    Note: To help speed things along, names.txt has already been created for you.
18. At the top of the script, replace the names list with the following code:
    names = []
    with open('names.txt','r') as file:
    for line in file:
         line = line.strip()
         names.append(line)

    

    This code snippet creates an empty list called names. The next line of code opens names.txt for reading and gives us a handle to the open text document called file. Then, a for loop extracts text from the file handle line by line. Each line is stripped (the newline character is removed) and appended to the names list. Using "with open" is an efficient way to enumerate a file, as the operation closes the open file handle for you.
    
19. At the bottom of the script, add the following code:
    file = open("names.txt","a")
    file.write(name_requested + "\n")
    file.close()

    

    Before the script ends, we open names.txt for appending. We then write name_requested plus a new-line character to the file handle. Lastly, we need to close the open file. We could also have used the "with open" method for appending data to names.txt:
    with open(names.txt,"a") as file:
      file.write(name_requested + "/n")
    Note: We exit() the program if the value of add_name is anything other than Y or y. Thus, the script will end without writing to the file if the user does not answer Y or y.
    
    Here is what the code should look like now:

    

20. Press CTRL + S to save, then re-run the script. Try adding Zelda to the names list, then re-run the script again and search for Zelda.
    
    If you run into trouble, there is a file on the desktop called names_complete.py. The file contains the program you just created, plus additional comments. As a scripter and developer, adding comments to make your program understandable to others (and your future self) is always a good idea! In Python, there are two kinds of comments:
    """
    This is a
    multiline
    comment
    """
    
    #This is a single line comment
    Comments do not need to describe every line of code in detail. Instead, they should give a general overview of what a given code block does (or is supposed to do).
21. Close vscode.

Summary

Congratulations! You have completed the guided portion of the lab. You have briefly covered a lot of Python: integers, strings, operators, slicing, lists, for loops, if/else causes, and even file handling. I hope this lab inspires you to learn more Python syntax and start writing your own tools in Python. Writing tools and automating tasks are the best ways to learn Python and grow as a technical professional.

Note: W3schools is an amazing resource for learning Python syntax ( https://www.w3schools.com/python/default.asp).

If you found this lab too difficult, or if you would like to go a bit deeper into the syntax used in this lab (and more), check out Intro to Python ( https://app.cybrary.it/browse/course/python).